Experimental Results

Ideally, any fertilization strategy should be shown to be effective before it is publicized and promoted. Research needs to validate or compare alternative strategies under controlled conditions. The University of Nebraska began a study to compare approaches to fertilizer recommendations in 1973 (Olson, et al., 1982). The study was designed to answer three questions:

 1. How do the soil test values compare‏  

2. How do the recommendations compare‏  

3. How do the recommendations perform‏

In this study, five Nebraska sites were selected. Plot areas were sampled, and soil samples were divided and sent to five soil testing laboratories, including the University. The samples were handled so that no laboratory, including the University, knew their recommendations were to be used in an experiment. Each laboratory received a request for a fertilizer recommendation to grow a given crop at a realistic yield goal for the area. This approach was used to reflect the normal fertilizer recommendations provided to any grower who requests soil tests and recommendations. This procedure eliminated dealer-producer contact which might alter the suggested program.

All recommended nutrients were assumed to be needed and were applied. All fertilizers were broadcast and incorporated prior to planting. This procedure was continued for 10 years at the same locations. Soils from the different plots of a given laboratory were resampled each year before the next cropping season and then submitted to the laboratory which made the initial test and fertilizer recommendations. Fertility recommendations were followed every year for each laboratory.

Comparisons of soil test levels between laboratories could only be made the first year because fertilizer programs differed after this. In general, variation was not too large in soil test values reported. The difference in fertilizer recommendations mentioned in the introduction is based on interpretation of the laboratory results and not in the analytical ability of the laboratories.

While this study was designed to compare specific laboratory recommendations, the data can be coded by fertilization strategy. The data has been arranged to compare the two main strategies used by soil testing laboratories: deficiency correction and the maintenance-balance approach. As expected, fertilizer recommendations and costs varied widely. There were no significantly different yield levels between laboratories at any location (Table 10.1). Data has been averaged for a seven-year period for four of the locations where corn was the test crop (Table 10.1).

Location North Platte   Clay Center   Mead   Concord  
Yield Goal 170 bu. Corn  

170 bu. Corn 

200 bu. (1976-80)

  170 bu. Cornb   90 bu. Cornc  
Fertilizer concept M+B DCa M+B DC M+B DC M+B DC
Fertilizer cost/acre $54 $26 $56 $30 $62 $35 $27d $12d
Yield - bu/acre 167 167 186 189 152 154 83d 84d

aDC = deficiency correction, M+B = maintenance plus balance

bEight-year average

cDryland

dDrought in one year. Number is average yield of cost for seven years since fertilizer was applied all seven years.

Table 10.1.  Comparison of fertilizer costs and grain yields for laboratories using the maintenance plus balance or deficiency correction approach.

Data from the North Platte location shows that the phosphate, sulfur, and micronutrient recommendations for the maintenance strategy are much greater than those of the deficiency correction approach (Table 10.2). The yields were the same regardless of fertilizer treatment, indicating that fertilizer applied to effect a more idealized crop production environment did not cause a sufficient crop response to pay for the added fertilizer. It is evident that if the extra phosphate, micronutrients and sulfur had been sufficiently deficient to affect yield increases, the result would be different, especially after seven years.

Laboratory A B C D E (UNL)
fertilizer concept M + B M + B M + B M + B DC
Nitrogen 1246 1445 1690 1310 1225
Phosphate (P205) 495 355 260 451 0
Potash (K20) 30 170 0 0 0
Magnesium 30 125 5 0 0
Sulfur 91 115 490 0 0
Zinc 12 15 52 0 5
Iron 3 1 0 0 0
Manganese 6 3 5 0 0
Boron 4 0 3 0 0
7-year average corn yield 168a 168a 166a 166a 167a1
           

1Yields followed by the same letter are not significantly different at the 5 percent level of probability

Table 10.2.  Total nutrients applied by five soil testing laboratories over seven years at North Platte (pounds per acre).

Another consideration of the deficiency correction concept for soil test interpretation is related to the environment. Wind and water erosion removes soils and nutrients from a field. These nutrients find their way into lakes and streams and cause excessive algae growth. High levels of nitrate-nitrogen in well water are currently a concern in parts of Nebraska. In this respect, a strategy that limits nutrient applications to only those required by the crop can limit the potential for nutrient losses from soil erosion or from leaching of nitrate. In terms of global resources, it is a tremendous waste of resources to transport nutrients around the country, especially if they don’t increase yields and have the potential to cause environmental problems.